Marc A. Meyers

Bio: Marc A. Meyers is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Deformation (engineering) & Dislocation. The author has an hindex of 85, co-authored 487 publications receiving 36646 citations. Previous affiliations of Marc A. Meyers include University of California & Instituto Militar de Engenharia.

Reaction synthesis/dynamic compaction of titanium carbide and titanium diboride

Abstract: Titanium carbide and titanium diboride were synthesized by self-propagating combustion reactions and densified by a high-speed forging technique, in which a hammer strikes the hot and porous ceramic (T - 2O0O0C) at velocities in the range 5 - 15 m/s. Compacts with densities higher than 96% of the theoretical density and diameters close to 10 cm were produced by this method. Optical and scanning electron microscopy of the consolidated material were carried out, revealing a grain size of approximately 44 pm for Tic and 15 pm for TiB2; these grains were equiaxed, indicating that crystallization (or recrystallization) occurred after the densification process. The quasistatic and dynamic compressive strengths were established and compared to those of hot-pressed material produced by CERCOM. For TiB2, the strengths were approximately 1.8 GPa, while for Tic the quasistatic (109 S-') and dynamic (102 S-') strengths were 1.7 and 2.2 GPa, respectively.

Martensitic transformation induced by tensile stress pulses

Abstract: Etude de la cinetique de transformation martensitique induite par une impulsion de contrainte produite par la reflexion d'une onde de choc compressive sur une surface libre dans les alliages Fe-32% Ni-0,035% C et Fe-22,5% Ni-4% Mn (% en poids). Influence de la variation de temperature ou de la duree de l'impulsion a pression constante. Determination des parametres cinetiques et energetiques de la transformation martensitique

Stresses Induced in Iron-Ore Pellets by Hydrogen Reduction

Abstract: The objectives of this investigation were (a) to determine the effect of reduction temperature on the strength of iron ore agglomerates and (b) to develop enhanced understanding for the cracking associ-ated with reduction. Iron-ore agglomerates from two sources (Samarco Mineracao and Bethlehem Steel) were reduced in a hydrogen atmosphere at temperatures varying from 873 K to 1373 K at intervals of 100 K and times varying from 30 to 300 minutes. The compressive strength at the ambient temperature of the pellets was determined after the various reduction treatments by using a piston-and-cylinder testing technique and computing the energy required in crushing them. The highest strength, at a specific level of reduction, was found after reduction at 1073 K, for both the Samarco and Bethlehem pellets. Profuse cracking of the pellets was observed after reduction. These cracks led to a weakening of the pellets. A mechanism for reduction-induced cracking, based on internal stresses due to volume changes produced by the chemical reactions, is presented.

A Model for Dislocation Generation in Shock-wave Deformation

Abstract: A model for dislocation generation in shock-wave deformation is described. Contrary to earlier models proposed by Smith (1958) and Hornbogen (1960), this model does not require the dislocations to move with the shock front; therefore no supersonic dislocations are needed. Dislocations are homogeneously nucleated at the shock front by the deviatoric component of the applied stress pulse; after generation, they are left behind, organizing themselves into more stable arrangements. Dislocation generation may also take place at the rarefaction part of the wave; however the mechanism is thought to be the conventional multiplication mechanism in this part of the wave. Dynamical considerations for nickel show that dislocations moving at velocities higher than the transverse sound velocity would lead to exceedingly high temperatures; experiments show that such is not the case. What renders the proposed model especially attractive is that, for the first time, quantitative predictions of residual dislocation densities are made possible. Accordingly, calculated dislocation densities are compared to observed densities for nickel reported in the literature.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Using Multivariate Statistics

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Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load

Abstract: The tensile strengths of individual multiwalled carbon nanotubes (MWCNTs) were measured with a “nanostressing stage” located within a scanning electron microscope. The tensile-loading experiment was prepared and observed entirely within the microscope and was recorded on video. The MWCNTs broke in the outermost layer (“sword-in-sheath” failure), and the tensile strength of this layer ranged from 11 to 63 gigapascals for the set of 19 MWCNTs that were loaded. Analysis of the stress-strain curves for individual MWCNTs indicated that the Young's modulus E of the outermost layer varied from 270 to 950 gigapascals. Transmission electron microscopic examination of the broken nanotube fragments revealed a variety of structures, such as a nanotube ribbon, a wave pattern, and partial radial collapse.